Nearly 1 in 10 Americans uses a fish oil supplement regularly, making it one of the most popular supplements in the U.S., say researchers at the National Center for Health Statistics. Americans spend more than $1 billion each year on fish oil. Many hope the supplements will improve joint health, sharpen the mind and prevent heart disease and strokes. But data about the health benefits of fish oils is inconsistent and often inconclusive.

Fish oils contain two types of omega-3 fatty acids called EPA and DHA (eicosapentaenoic acid and docosahexaenoic acid). Our bodies can’t make these fatty acids, so we have to consume them. Good sources of EPA and DHA are oily fish such as mackerel, herring and sardines. Some non-animal foods, such as pumpkin seeds and walnuts, contain a different kind of omega-3 fatty acid, alpha-linolenic acid, which the body converts to DHA and EPA. These fatty acids are transformed by the body into anti-inflammatory chemicals called resolvins.

Omega-3 fatty acids have been touted as preventing heart disease since at least the 1970s, when researchers postulated that Inuit people in Canada and Alaska had lower rates of heart disease because they ate a diet rich in fish. That theory was later debunked, and researchers said the Inuit probably have more heart disease than the earlier scientists accounted for. But claims about fish oil supplements have persisted.

The data, however, is unclear. I read through nearly 30 studies on fish oils published in the last eight years and found only two that showed the supplements were better than placebos at improving the health of those that took them.

There is some evidence that oily fish and fish oil supplements may ease the pain and swelling of rheumatoid arthritis. One study found that users had less joint pain in the morning and reported that their joints felt less tender. But many studies looking at the effects on rheumatoid arthritis were small and involved users taking multiple supplements.

Other studies looked at the effect of fish oils on blood pressure. While some reported the supplements lowered blood pressure, these also were small studies. Other studies found that fish oils had no impact on blood pressure.

There is better evidence that fish oils lower triglyceride levels, but some of these studies relied on high doses — up to 4 grams per day — and on combining fish oils with other medicines such as statins. Prescription fish oil supplements containing high doses of omega-3 fatty acids have been approved by the Food and Drug Administration, but these can cause side effects such as prolonged bleeding.

After a safety review of fish oils in 1997, the FDA said that people should take no more than 3 grams of EPA and DHA per day. Side effects of taking fish oils at high doses range from the annoying — nausea and a fishy burp — to the dangerous — prolonged bleeding caused by a disruption in the clotting system.

Those with risk factors for heart disease and stroke may be disappointed to learn that a study by Italian scientists found fish oils did not prevent these illnesses. More than 12,000 patients were tracked in a study published in 2013 . Half of the study participants received fatty acid supplements while the rest were given a placebo. Ultimately, those given supplements did not fare better than those given olive oil, which served as the placebo. Also, this seems like a strange choice for a placebo given some evidence that the fats in olive oil also lower the risk of heart disease.

Increased cancer risk?

In the past few years, research has emerged showing fish oils may increase the risk of prostate cancer. In 2013, researchers at the Fred Hutchinson Cancer Research Center in Seattle found that people who take high-dose fish oil supplements or eat a lot of oily fish had a 43 percent increased risk for prostate cancer and a 71 percent increased risk for aggressive prostate cancer.

But even that finding is inconsistent with other studies. This summer, scientists in Canada published a systematic review of 44 studies looking at fish oil supplements. They concluded that the “current evidence is insufficient to suggest a relationship between fish-derived omega-3 fatty acid and risk of [prostate cancer]." They suggested that more research is needed.

Should you take a fish supplement?

The American Heart Association recommends eating two servings of oily fish a week. A serving is 3.5 ounces of cooked fish or approximately three-quarters of a cup of flaked fish. Research shows that most Americans eat less than this and opt for fish such as shrimp and canned tuna, which contains fewer omega-3 fatty acids than fish such as mackerel and salmon.

For those with heart disease, the AHA recommends 1 gram of omega-3 fatty acids per day and 2 to 4 grams of fish oil for those with high levels of triglycerides. Always tell your doctor about dietary supplements. Fish oils can interact with the blood-thinning medicine warfarin (also known as Coumadin). Pregnant women should eat no more than 8 ounces of albacore tuna per month and avoid fish high in mercury including king mackerel and swordfish.

Debunked is your go-to site for demystifying science and medicine. Send your questions and conspiracy theories to syasmin@dallasnews.com or tweet me at @DoctorYasmin. I'm a medical writer at The Dallas Morning News and a professor at the University of Texas at Dallas. I worked as a medical doctor and disease detective before training as a journalist.